Theoretical calculations of minimum energy structures and thermodynamics for the formation of cyclic amidines and imidates by intramolecular nucleophilic addition/elimination reactions

Abstract

Theoretical calculations of minimum energy structures and thermodynamic terms using SCF theory with thermodynamic and solvation corrections have been made of the cyclisation of 1-amino-8-(acetylamino)naphthalene to give 2-methylperimidine with the liberation of water and of the related reaction of 1-hydroxy-8-(acetylamino)naphthalene to 2-methylnaphtho[1,8-d,e][1,3]oxazine. The calculations predict that in the gas phase the former reaction is strongly thermodynamically favourable (ΔG = –37.9 kJ mol^–1, ΔH = –0.6 kJ mol^–1 and TΔS = +37.3 kJ mol^–1 at 298.2 K) whereas the latter is much less favourable (ΔG = –2.0 kJ mol^–1, ΔH = +31.1 kJ mol^–1 and TΔS = +33.1 kJ mol^–1 at 298.2 K). The results are in qualitative agreement with experimental observations for the reactions in solution. Reasons for the different behaviour of the two reactions are discussed.